1. Field of the Invention
This invention relates to an image forming apparatus using electrophotographic process, and a method of estimating the amount of toner consumed by that image forming apparatus.
2. Description of the Related Art
In the developer of an image forming apparatus using electrophotographic process, a two-component developing agent consisting mainly toner particles (carbon particles, etc.) and carrier particles, is typically used. For example, in the case of an image forming apparatus that uses the two-component developing agent, the toner particles are consumed in the developer when forming an image, and the density of the toner with respect to the carrier particles, or in other words, the toner density decreases. In order to maintain the image quality, it is necessary for the toner density to be constant, and therefore it is necessary to supply toner particles periodically as the amount of toner particles decreases in order that the toner density remains constant. A magnetic permeability sensor is often used as a toner-density-measurement means of measuring the density of the toner particles. As the amount of toner particles decreases the magnetic permeability becomes high, or in other words, the magnetic permeability sensor detects magnetic permeability, and as a result detects a decrease in the amount of toner particles.
Since the sensor is expensive, a method of estimating the amount of toner particles consumed is used as a means of detecting the toner density without using this kind of sensor. A method of estimating the amount of toner particles consumed is disclosed in Japanese laid-open publication No. S58-224363, in which the individual pixel signals that make up the printing pixels when the image is formed are counted, and since the count is proportional to the amount of toner particles consumed, the amount of consumed toner is found from that ratio. The printing pixels referred to here are the pixels to which toner particles should be adhered.
However, as described in the publication mentioned above, the actual amount of consumed toner increases as the number of printing pixels increases, and the amount of consumed toner is not strictly proportional to the number of pixels. Depending on the type of image, the amount of consumed toner may differ even when the number of pixel to which the toner adheres is the same. For example, the amount of consumed toner per one pixel tends to become larger in line images than in solid images for which a specific area is filled in.
More particularly, in the case of the example of the printing surface having 4×4 pixels as shown in FIG. 3A, when toner is made to adhere (be fixed) to a square-shaped pixel 302, the toner adheres to the pixel in a circular shape 301 or elliptical shape. This is because that when filling in the image, it is necessary to have toner adhere to a contact point 304 between the pixels that come in contact in the diagonal direction (in other words, point-contacting pixels) 303. When the toner is adhered to an area smaller than the size of the pixel, the area around the contact point 304 is not sufficiently filled in.
Since the toner is adhered to an area larger than the size of the pixel, there are four leaf-shaped overlapping areas 305 when four pixels are filled in as shown in FIG. 3B, for example. Therefore, the amount of consumed toner does not become twice the amount as when filling in the two pixels shown in FIG. 3A, and results in an error in the amount of consumed toner. Also, in the case where toner is adhered to just one pixel 307 (isolated pixel) without adhering toner to the surrounding pixels as shown in FIG. 3C, it is generally known that the area to which toner adheres 308 is less than the area of the pixel 307. This is because in the exposure process of removing the electric charge from the portion where light comes in contact with the photosensitive drum, the time that the light goes ON/OFF is very short, so the charge cannot be sufficiently removed. The isolated pixel is also a factor in causing large error in the amount of consumed toner.
Error-removal processing for doing away with the difference between the estimated amount of consumed toner and the actual amount of consumed toner is actually performed a few times on several pages of printing. This error-removal processing is a process in which a specified pattern form measuring the density is printed internally, and the amount of consumed toner is adjusted by optically measuring the density of the printing.
However, when performing the error-removal process, there is a problem in that the printing process must be stopped, and particularly when the error-removal process is performed frequently, the number of printed pages per unit time, which is a measure of the performance of the image forming apparatus, greatly drops.
Moreover, there is a problem in that the costs related to each of the steps required in the error-removal process increase the overall cost of the image forming apparatus.